Different concentration probes are known in the technical field. Besides so-called step change sensors, whose characteristic curve has a significant change at lambda=1, said change being used for determining the value lambda=1, so-called wide-band lambda probes exist. A wide-band lambda probe and a circuit arrangement for operating such a wide-band lambda probe emanate from, for example, the book publication of “Bosch Kraftfahrttechniches Taschenbuch” (“Bosch Automotive Handbook”), 25th edition, October 2003, page 134. Such a probe as a multilayer ceramic component essentially consists of a combination of a conventional concentration probe (Nernst probe), which acts as a galvanized cell, as well as a limiting-current or “pump” cell. A voltage is applied from outside to the pump cell. If the voltage is large enough, a so-called limiting current arises, which is proportional to the difference in the oxygen concentration of both sides of the probe. Oxygen ions are transported with the current while depending on polarity. By means of the circuit arrangement, which represents an electronic closed-loop control circuit, it is assured that via a very narrow diffusion gap the concentration probe of the pump cell is always supplied with exactly that much oxygen from the exhaust gas for the condition lambda=1 to prevail at it. When there is an excess of air in the exhaust gas in the so-called lean range, oxygen is pumped out of the cell. When the residual oxygen content of the exhaust gas is too low in the rich range, oxygen is supplied to said probe by reversing the pump voltage. In so doing, the pump current forms the output signal of the probe.
The balancing of such a planar wide-band lambda probe takes place via a current divider. For this purpose, an adjustable resistor is disposed in a plug of the probe, said plug being embodied, for example, according to the German patent DE 201 06 750 U1. Said resistor is connected in parallel to a measuring resistor, which is preferably part of a control unit. In the case of diffusion barriers with a high amount of limiting current, the balancing resistor is left at low impedance. When the amount of limiting current is low, the resistance is increased as a result of a laser performing an incision into said balancing resistor. The pump current concentration characteristic curve is rotated by means of this resistor so that beside the balance point all other points also lie on the setpoint value characteristic curve.
The probe is connected via corresponding cables to an analysis circuit device, which, for example, is part of a control unit.
A line break in the feed cable to the probe can not be readily detected by sensor elements and switching devices, which are known from the technical field. According to the specifications of the On-Board Diagnostics II (OBD II), a continued monitoring of all of the emission-related components is now required. In particular lambda probes and the probe cables of such lambda probes also have to be continually monitored, for example clocked each 500 ms. At the same time, a check also must be made to determine whether a cable break exists, which can in principle occur during the operation of a motor vehicle.
Therefore, the task underlying the invention is to put forth a device for checking the operability of a sensor element, in particular a wide-band lambda probe, by which such a cable break can be detected.